The invention relates to an apparatus for the in-line cropping and/or chamfering, using one or more cutting devices, of a plastic tube moulded by extrusion, which apparatus is suitable for siting in an extrusion line for the continuous moulding of such a plastic tube, comprising means for positioning a tube section with respect to a cutting device or the cutting devices and at least comprising per cutting device two deforming rollers for deforming the tube section on which the cropping and/or chamfering operation has to be carried out, the apparatus comprising moreover a rotatably arranged combination of a face cam having a groove composed of a plurality of levels and a rotor disc for controlling the movement of the deforming rollers and the cutting device or the cutting devices in carrying out the cropping and/or chamfering operation.
Such an apparatus is known from the laid-open Dutch Patent Application 7705659 of the Applicant.
The rotor disc mentioned above in the preamble is a disc having a number, for example four, of oscillating pieces each having a composite cam-follower roller. The movement of the cam-follower rollers of the composite cam-follower roller in the respective grooves of the face cam controls the radial movement of the deforming rollers and the cutting devices.
In the above application three pairs of deforming rollers and three cutters, which are all symmetrically positioned around the tube, are shown. A cutter has, in axial direction of a tube, one deforming roll on either side.
In such an apparatus, a continuously moulded plastic tube is divided into pieces as required by using one or more cropping tools, while a suitable chamfering is obtained by using one or more chamfering tools.
To obtain a reproducible cropping and chamfering result, the tube is positioned, prior to such an operation, with respect to the combination of tools, which will hereinafter be termed in its totality as cutting devices, after which the tube is deformed with the aid of deforming rollers in order to provide the tube with a rigidity such that it will no longer deform when acted on by the cutting devices. The apparatus for carrying out the cutting operation moves while carrying out the operation at the same axial speed as that with which the tube is moulded.
The movement of the deforming rollers on either side of a cutting device and the cutting devices is controlled by means of a combination of a face cam and a rotor disc, the face cam comprising a groove which is subdivided into two levels. One level regulates, by means of a first cam-follower roller, the movement of the deforming rollers towards the surface of the tube and the degree of deformation of the tube, while the groove in the second level provides, by means of a second cam-follower roller, for the movement of a cutting device.
The groove is composed in its generality of a number of identical sections adjoining one another, each of the identical sections providing the possibility of causing the deforming rollers and the cutting devices to carry out a complete movement and operation cycle.
To cause the first and second cam-follower rollers on the rotor disc to move in the composite groove of the face cam, use is made, in the apparatus described above, of a difference in circumference of the two discs.
By making the circumference of the two discs different, a difference in rotary speed of the two discs will be obtained when the two discs are driven by the same motor, as a result of which the cam-follower roller will move in the groove with respect to an initial position. Because all cam-follower rollers pass through one of the groove sections completely, the deforming rollers and the cutting devices return to their initial position, at which moment a coupling between the pulleys thereof is released and the device is moved back in axial direction to its starting position. A new operation cycle is then carried out upon the next energizing of the coupling which couples the drive-pulley of the face cam to that of the rotor disc.
A known device of this type has the disadvantage that, as a result of the constant difference in rotary speed which occurs between the rotor disc and the face cam as a consequence of their fixed difference in diameter, the movement of the deforming rollers and the movement of the cutting devices will always take place at one and the same speed and over the same distance.
If, therefore, at the one hand tubes having small diameter and large wall thickness have to be cut, the movement of the cutting device will take up a disproportionately long period of time, as a result of which the bed of the cropping and/or chamfering apparatus will have to be disproportionately long and requires unnecessary much energy.
At the other hand, upon cutting of tubes having a large diameter and a small wall thickness, the cutter will pierce the wall with too high speed which results in high forces.